Apparatus for imparting electrostatic charges in electrophotography



June 30, 1959 v STRAUGHAN 2,892,973

APPARATUS FOR IMPARTING ELECTROSTATIC CHARGES Fi led Jan 26, 1955 ELECTROPHOTOGRAPHY 2 Sheets-Sheet l INVENTOR.

VIRGIL E. STRAUGHAN LAWRENCE l. FIELD ATTORNEY June 30, 1959 v. E. STRAUGHAN 2,892,973

APPARATUS FOR IMPARTING ELECTROSTATIC CHARGES" IN ELEC HY TROPHOTOGRAP Filed Jan. 26, 1955 2 Sheets-Sheet 2 IN VEN TOR. VIRGIL E. STRAUGHAN BY LAWRENCE I. FIELD ATTORNEY United States Patent APPARATUS FOR IMPARTIN G ELECTROSTATIC CHARGES IN ELECTROPHOTOGRAPHY Virgil E. Straughan, Euclid, Ohio, assignor, by mesne assignments, to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware Application January 26, 1955, Serial No. 484,214

3 Claims. (Cl. 317-262) This invention relates to electrostatic printing. More particularly, it is concerned with a device for imparting an electrical charge to the surface of a photoconductive material to ultimately secure thereon an electrostatic image from which a visual image may be developed and ultimately transferred to a suitable permanent record material.

The art of electrophotography is of comparatively recent development and one interesting aspect thereof is embodied in US. patents issued to Chester F. Carlson, namely, 2,221,776; 2,277,013; 2,297,691; 2,357,809 and 2,551,582. The prior art devices therein disclosed, involve the use of a photoconductive element which is electrostatically charged and exposed to light either simultaneously with charging or subsequently thereto to produce a desired latent image thereon. The photoconductive element bearing a latent electrostatic image is then contacted with a dust or powder which is attracted to the charged portion of the plate and the dust or powder image is finally transferred to an adhesive bearing paper. In such devices, the photoconductive layer is charged either by friction as for example by rubbing vigorously with a soft material such as a cotton or silk handkerchief or fur as shown specifically in Patent 2,297,691 or by rubbing with a soft, plush-covered roller as in Patent 2,357,809 or by connecting the photoconductive plate to one terminal of a high voltage source the other of which is connected to a metal plate positioned parallel to and in spaced relation from the photoconductive surface to be charged, whereby an electric field is established and the surface is charged upon illuminating same while the potential is applied across the plate.

In a copending application of Edward F. Mayer, 380,- 285, filed September 15, 1953, there is disclosed an improvement over the photoelectric photographic devices of Carlson in which the unitary electrophotographic element described in Carlson 2,277,013 is divided into two separableelements. The method of producing a visual reproduction described in the aforesaid copending Mayer application Serial No. 380,285, comprises directing a light image of the object through the transparent layer of electrically conductive material and thence through the contiguous high resistance photoconductive layer toward the adjacent but separable layer of electrically insulating material backed by a supporting metal layer. A positive electric potential (or negative electric potential, if a negative electric image is desired) is applied to the supporting metal layer with respect to the transparent layer of electrically conductive material, thus establishing on the layer of insulating material an electrostatic reproduction of the light image. There is thereafter applied to the surface of the insulating material bearing the electrostatic reproduction of the image, after it has been separated from the previously adjoining photoconductive material, a dispersion of powdered carbon with the resulting formation of a visual reproduction of the aforesaid light image on the surface of the separated insulating material.

2,892,973 Patented June 30, 1959 ice there is described a modification of the method and apparatus of his earlier application Serial No. 380,285. In the later filed application, in the method disclosed, the latent electrostatic image is caused to be formed on the photoconductive layer instead of on the electrically insulating layer, and is thereafter developed in any suitable fashion.

I have discovered an improved apparatus for imparting an electrostatic charge to a photoconductive surface such as that employed by Carlson or the photoconductive surface described in the applications ofMayer, above identified, or indeed for charging any photoconductive surface. Briefly, my invention involves the use of a bladeshaped wand resiliently mounted so as to press'lightly against the photoconductive surface. The wand is incorporated in an electric circuit in which it is connected to one terminal of a potential source the other terminal of which is electrically connected to the conducting sub strate of the photoconductor member.

One object of my invention is to provide an apparatus whereby a more uniform electrostatic charge may be imparted to a photoconductor than the charge imparted by prior art devices.

Another object of my invention is to provide a relatively compact apparatus in which a wide variety of materials having the desired electrical characteristics may be employed in a simple electric circuit to effectively charge a photoconductor member.

Another object of my invention is to provide a charging apparatus with relatively few moving parts and which is compact and simple.

Another object of my invention is to provide an inexpensive charging apparatus which might be incorporated in any device requiring a means for producing anelectrostatic charge on a high resistance surface.

Figure l is a view in section of an apparatus for charging a flat photoconductive surface. Figure 2 is a View in section of the apparatus of Figure l modified to charge a cylindrical photoconductive surface and Figure 3 is a and a high resistance photoconductive layer 16. This photoconductive layer is selected from materials having a resistivity in the dark of 10 ohm-cm. or greater, and a resistivity when exposed to the maximum light intensity of 10 ohm-cm. or less. These values are arbitrary and depend upon the particular operating conditions. Materials such as selenium, arsenic selenide, zinc-cadmium sulfide and many other known compositions having the desired properties may be employed as the photoconductive layer 16. The layer 16 should be continuous and like layer 14 it may be advantageously deposited on the layer beneath by vapor deposition. Photoconductive plate 10 is connected to a source of potential such as one pole of battery 36, through a lead to layer 14. The other pole of the battery is connected to the charge applying means 20.

The charging means 20 comprises a conductive bladeshaped charge applicator or wand 22 one end of which is connected to battery 36 and the other end of which is covered with a layer of resilient rubber 28. As shown in the figure the rubber is coated with a layer of. an air drying metallic paint 30 and further covered with a sheet of cellophane 32. Lead 33 connects the layer of metallic paint to wand 22. Thus, layers 14, 16, 30 and 32 form a sort of capacitor in which layers 14 and 30 have impressed between them the potential supplied by battery 36. With 900 volts applied across layers 16 and 32, charges of 100 to 150 volts have been readily applied to the surface of photoconductive layer 16. The charge applicator is drawn lightly across the surface of the photoconductive layer 16 and charge is leaked through layer 32 and onto layer 16. The positive 11 is imaged onto photoconductor 16 by means of illumination supplied by a source 50.

In Figure 2 there is shown a slightly different form of the apparatus of Figure 1. In this embodiment an applicator 20 which does not materially differ from that shown in Figure 1 is applying a charge to the surface of a photoconductor in the form of a drum The rotating drum 10' is covered with a thin layer of a photoconductive material which is to be electrostatically charged. The drum is formed of a transparent materia'l such as a glass cylinder 12' and is preferably hollow as shown. This cylinder is first coated on its outer surface with a layer of a transparent conductive material 14' such as a thin layer of tin oxide, or of evaporated gold with bismuth additions, or other conventional metallic conductors which are transparent in thin layers. Alternatively the drum may also be formed of any transparent plastic with the transparent conductive coating upon its outer surface. A third alternative construction comprises a conductor chemically deposited upon the surface of a glass or plastic cylinder. The specific cylinder construction forms no part of my invention, except for the particular materials chosen, which must possess the appropriate electrical properties to match with those of my improved charging device in the desired fashion.

The transparent conducting layer 14 is chosen from materials which are moderately transparent to visible light, i.e. having a transparency of 50% to white light and are of fairly low resistance, for instance between 100 and 1000 ohms per square surface resistance. The aforesaid transparency requirement is arbitrary and may be greater or less if lower intensity or higher intensity light is employed, to expose the photo conductive surface. Over the transparent conductive material, there is formed a layer of high resistance photoconductive material 16. This photoconductive layer should have a resistivity in the dark of about 10 ohmcentimeters or greater, and a resistivity when exposed to the maximum light intensity of about 10 ohm-centimeters or less. These resistivity values are arbitrary and may be varied as other factors are altered to meet various operating conditions. Selenium, arsenic selenide, zinc-cadmium sulfide and many other substances all of which possess the aforementioned combination of properties have been found suitable. The photoconductive layer should be continuous and is advantageously deposited on the cylinder after transparent conducting film has been formed thereon. While any method of coating may be employed, a preferred method of applying the photoconductive layer is by vapor deposition in a vacuum. The hollow drum 10 is supported by suitable hearings in a framework (not shown) supported on a base 18. Charging means 20 is positioned so that it contacts the photoconductcr surface of drum 10.

As shown in Figure 2, the charging means 20 includes a conductive blade-shaped charge applicator or wand 22 attached to a hinge 24 supported on base 18 and insulated therefrom by block 26. The free end of the hingemounted wand 22 is covered with a layer of resilient rubber 28 coated with a layer of an air drying metallic (silver) paint 30 and further covered with a sheet of a semiconductive material, such as cellophane 32. The metallic paint layer 30 is electrically connected to Wand 22 'by a lead 33. Spring 34 insulated from the base 18 is attached to the base by means of a pad of insulation 27 and is attached to the conductor so as to continually urge the free end of the charge-applying wand against the drum 10. One pole of a battery 36 is connected to a conductor 22 and the other terminal is connected to conducting layer 14 of the drum. With 900 volts applied across films 16 and 32, charges of to volts were applied to the photoconductor surface of rotating drum 10 when rotated a single revolution. Instead of sheet cellophane layer 32 and rubber layer 28, conductor 22 may be covered directly with a layer of conductive rubber of high resistance and made to contact the photoconductor surface. Other changes in design may be made by those skilled in the art without departing from the scope of my invention. Thus, the tip of blade 22 may be shaped to conform to a portion of the surface of drum 10 so as to spread the charge more uniformly and to minimize wear on the photoconductive layer of drum 10.

Figure 3 shows the apparatus of Figure 2 schematically incorporated in a high speed electrophotographic apparatus. In Figure 3 there is shown a drum 10' supporting a photoconductive layer as previously described. Charge applicator 20 is similar to that shown in Figures 1 and 2 except that the wand is now covered with a single layer 29 composed of a semiconductive material having an appropriate resistance, chosen to fit the particular operating conditions. Semiconductive materials having resistivities on the order of 1-1000 megohms-cm. such as plastics and rubbers have been found suitable for the purpose. As shown in Figure 3, the means 50 may be provided either within drum 10 or externally thereof for projecting the light image on the photoconductive surface supported on the drum. The surface after exposure and charging, or charging and exposure as the case may be, is rotated into contact with an applicator 41 by means of which a powdered material is applied to the surface of the drum where it precipitates in response to the latent electrostatic charge pattern carried on the drum. The powder may advantageously be in the form of a dispersion of carbon particles in a suitable liquid contained in tank 42. On further rotation of the drum 10 the powder image is transferred to a permanent record material 46 which contacts drum 1.0 through roller 47. Further rotation of the drum brings the photoconductive layer to a cleaning means 48 where it is restored to a condition for further use.

From the foregoing description of the apparatus the manner in which same is operated should be evident. In the embodiment shown in Figure 1 the conductive layer of the photoconductive element is connected to one terminal of a source of potential and the charge applying wand is connected to the other. To charge the photoconductor, the Wand is passed by hand one or more times across the photoconductive element is intimate contact therewith. Obviously, either the charge applying means may be moved relative to a stationary plate, or the photoconductive plate may be moved relative to a stationary charge applying means, or both may be moved relative to each other.

In the apparatus shown in Figure 2, after the roll 10' and charging means 20 have been mounted on base 18 one terminal of battery 36 is connected to the wand 22 and the other terminal is connected to the inner surface of the transparent conducting film 14'. When so connected, the photoconductive layer 16' and cellophane sheet 32 become the dielectric of a sort of condenser betveen the transparent conductive film 14 and the metallic paint layer 30. When a sufiicient potential is applied across this dielectric, a charge leaks from the charge applicator blade 22 onto the surface of the photoconduct-ive layer 16'. With about 900 volts potential applied from a DC. battery source, a charge of between 100 and '150 volts leaks onto the photoconductor. The effective electrostatic charge produced is a function of the photoconductors resistance and the magnitude of the potential applied.

A least three distinct possibilities exist relative to the sequence of exposure and charging of the photoconductive surface. Thus, a light source may be positioned either within the drum or an external light source may be employed to focus an image on the photoconductive layer 16. By appropriately positioning a shield, the light source may be directed to any desired area of the photoconductive element whether a plate as in Figure 1 or a drum as in Figure 2, or whatever its shape.

In one method of operation the illumination and charge application are made to occur simultaneously. This is effected by focusing the illumination so that it falls on the line of contact of the charge applying means 20 with the photoconductive surface layer of elements or 10'. The illuminated areas become conductive while the dark areas retain their relatively high resistance. The charge supplied by wand 22 flows through the illuminated areas of layer 16 and the transparent conducting layer 14 and these areas of the photoconductor are therefore discharged. In the non-illuminated areas of comparatively high resistance no current flows through layer 16 to dissipate charge imparted by wand 22, hence these areas remain charged and form the latent electrostatic image. This image may be developed as shown in Figures 2 and 3 by further rotation of drum 10' to a developing means 42, containing a suspension or dispersion of carbon black particles which form a positive corresponding to the image projected on the drum. Further rotation past a transfer mechanism permits the image to be removed to an absorbent belt or an adhesive-coated tape. Further rotation past a cleaning means prepares the drum for reuse.

Instead of simultaneous charging and image formation, the photoconductive surface may be exposed to the imageforming means before charging, provided the photoconductor layer is formed from a material having a sufiicient carrier life to render same photoconductive for at least the time required for an exposed area of the layer to have an appropriate charge applied thereto. In the apparatus of Figures 2 and 3, adjusting the speed of rotation of the drum permits this interval to be shortened or lengthened within the limits permitted by the material chosen for the photoconductive layer. Similarly dependent on the rate at which charge leakage on the drum occurs, the image-forming means may be positioned to form an image on a line or area of the drum after same has been charged.

It will be seen that I have provided a simple and compact means for applying a substantial electrostatic charge onto a photoconductive element in a more uniform manner than may be obtained by either the frictional (rubbing) method of the Carlson patents or the electrostatic charging apparatus disclosed in either of the aforesaid copending Mayer applications.

I claim:

1. An apparatus for imparting an electrostatic charge to an electrophotographic element which includes a photoconductive high resistance material secured to and supported on a conductive material which comprises: an electrically conductive blade, a layer of resilient insulating material covering one extremity of said blade, a film of a conductive paint covering said resilient insulating material, a lead constituting an electrical connection between said paint film and said blade, a thin layer of a semiconductive material covering the so-coated extremity of said blade, means to urge said covering material into intimate contact with the photoconductive material, a source of D0. potential, means connecting said source of potential to the conductive material secured to the photoconductive material and connecting said source of DC. potential to the conductive blade so that said potential is impressed across the photoconductive material and the blade covering material and means for moving said covered blade relative to said high resistance photoconductive material whereby a charge is leaked onto the surface of the high resistance photoconductive material.

2. An apparatus for wiping an electrostatic charge onto an electrophotographic element which includes a photoconductive high resistance material secured to and supported on a conductive material which comprises: a support, an electrically conductive blade, hinge means securing one end of said blade to said support, a layer of resilient insulating material covering the terminal portion of the free end of said blade, a conductive film covering said resilient insulating material, a lead constituting an electrical connection between said film and said blade, a thin layer of a semiconductive material covering the so-coated extremity of said blade, spring means urging the covered free end of said blade into intimate contact with the photoconductive material, a source of DC. potential, means connecting said source of DC. potential to the conductive material secured to the photoconductive material and connecting said source of DC. potential to the conductive blade so that said DC. potential is impressed across the photoconductive material and the blade covering material and means for moving said covered blade relative to said high resistance photoconductive material whereby a charge is leaked onto the surface of the high resistance photoconductive material.

3. An apparatus for wiping an electrostatic charge onto an electrophotographic element in the form of a drum consisting of a photoconductive high resistance material secured to and supported on an electrically conductive cylinder which comprises: an electrically conductive blade, a layer of resilient insulating material covering one extremity of said blade, a film of a conductive paint covering said resilient insulating material, a lead constituting an electrical connection between said paint film and said blade, a thin layer of a semiconductive material covering the so-coated extremity of said blade, means to urge said covering material into intimate contact with the photoconductive material, a source of DC. potential, means connecting said source of DC. potential to the conductive material secured to the photoconductive material and connecting said source of DC. potential to the conductive blade so that said DC. potential is impressed across the photoconductive material and the blade covering material and means for rotating said drum while said blade is urged into contact therewith whereby a charge is leaked onto the surface of the high resistance photoconductive material.

References Cited in the file of this patent UNITED STATES PATENTS 2,588,699 Carlson Mar. 11, 1952 2,693,416 Butterfield Nov. 2, 1954 2,701,764 Carlson Feb. 8, 1955 2,774,921 Walkup Dec. 18, 1956 FOREIGN PATENTS 539,861 France July 1, 1922 

1. AN APPARATUS FOR IMPARTING AN ELECTROSTATIC CHARGE TO AN ELECTROPHOTOGRAPHIC ELEMENT WHICH INCLUDES A PHOTOCONDUCTIVE HIGH RESISTANCE MATERIAL SECURED TO AND SUPPORTED ON A CONDUCTIVE MATERIAL WHICH COMPRISES: AN ELECTRICALLY CONDUCTIVE BLADE, A LAYER OF RESILIENT INSULATING MATERIAL COVERING ONE EXTREMITY OF SAID BLADE, A FILM OF A CONDUCTIVE PAINT COVERING SAID RESILIENT INSULATING MATERIAL, A LEAD CONSTITUTING AN ELECTRICAL CONNECTION BETWEEN SAID PAINT FILM AND SAID BLADE, A THIN LAYER OF A SEMICONDUCTIVE MATERIAL COVERING THE SO-COATED EXTREMITY OF SAID BLADE, MEAND TO URGE SAID COVERING MATERIAL INTO INTIMATE CONTACT WITH THE PHOTOCONDUCTIVE MATERIAL, A SOURCE OF D.C. POTENTIAL, MEANS CONNECTING SAID SOURCE OF POTENTIAL TO THE CONDUCTIVE, MEAND CONECTING SAID SOURCE PHOTOCONDUCTIVE MATERIAL AND CONECTING SAID SOURCE OF D.C. POTENTIAL TO THE CONDUCTIVE BLADE SO THAT SAID POTENTIAL IS IMPRESSED ACROSS THE PHOTOCONDUCTIVE MATERIAL AND THE BLADE COVERING MATERIAL AND MEAND FOR MOVING SAID COVERED BLADE RELATIVE TO SAID HIGH RESISTANCE PHOTOCONDUCTIVE MATERIAL WHEREBY A CHARGE IS LEAKED ONTO THE SURFACE OF THE HIGH RESISTANCE PHOTOCONDUCTIVE MATERIAL. 